The Suzuki-Miyaura reaction is a palladium- or nickel-catalyzed cross coupling between a boronic acid or a boronic ester and an organohalide or an organo-pseudohalide. (Miyaura, A. Chem. Rev., 1995) This cross coupling transformation is a powerful method for C—C bond formation in complex molecule synthesis. The reaction is tolerant of functional groups, and has become increasingly general and widespread in its use for coupling of organic compounds. (Barder, 2005; Billingsley, 2007; Littke, 2000; Nicolaou, 2005)
Boronic acids are notoriously sensitive to many common reagents. (Hall, 2005; Tyrell, 2003) It is therefore typical to introduce the boronic acid functional group during the last step of a building block synthesis. However, many of the methods for doing so (hydroboration, trapping organometallic reagents with trimethylborate, etc.) are intolerant to a variety of common functional groups, such as alcohols, aldehydes, ketones, alkynes and olefins. This makes the synthesis of structurally complex boronic acid building blocks quite challenging. In contrast, organostannanes are remarkably tolerant to a wide variety of reaction conditions and are routinely carried through multiple steps en route to structurally complex coupling partners. As a result, organostannanes have found widespread use in complex molecule synthesis (De Souza, M. V. N., 2006; Pattenden, G., 2002; Hong, B.-C., 2006) despite their well-known drawbacks including toxicity, high molecular weight, and byproducts that are difficult to remove. The ability to similarly carry protected boronic acids through multi-step synthetic sequences could substantially heighten their utility and broaden the scope of their applicability.
One area of research on the Suzuki-Miyaura reaction is the development of protecting groups for the boronic acid functional group. A compound that includes a protected boronic acid and another functional group can undergo chemical transformations of the other functional group without chemically transforming the boron. Removal of the protecting group (deprotection) then provides the free boronic acid, which can undergo a Suzuki-Miyaura reaction to cross-couple the compound with an organohalide or an organo-pseudohalide.
In one example of a boronic acid protecting group, each of the two B—OH groups is converted into a boronic ester group (>B—O—R) or a boronic amide group (>B—NH—R), where R is an organic group. The organic group can be removed by hydrolysis to provide the free boronic acid. Current data suggest that transmetalation between boronic acids and Pd(II) requires formation of an electronically-activated anionic boron ‘ate’ complex and/or a hydroxo μ2-bridged organoboronate-Pd(II) intermediate. Both mechanisms require a vacant boron p-orbital that is Lewis acidic. Bidentate ligands that contain strongly electron-donating heteroatoms are known to inhibit the cross-coupling of organoboron compounds, presumably by reducing the Lewis acidity of the sp2-hybridized boron center. Harnessing this effect, a few selective cross-couplings with boron-protected organoboranes that contain halogens have been reported recently. (Deng, 2002; Hohn, 2004; Holmes, 2006; Noguchi, 2007) Examples of protecting groups used in these selective reactions include pinacol esters (boronic ester) and 1,8-diaminonaphthalene (boronic amide). The heteroatom-boron bonds in these protected compounds tend to be very strong, however. The relatively harsh conditions required for cleaving these ligands typically are incompatible with complex molecule synthesis.
In another example of a protected boronic acid, the boron containing compound is converted into a tetracoordinate anion, such as [R—BF3]−, where R represents an organic group. Compounds containing these protecting groups are present as salts with a counterion, such as K+ or Na+. These anionic compounds are reported to be effective for inhibiting the reaction of boron during chemical transformations such as nucleophilic substitution, 1,3-dipolar cycloaddition, metal-halogen exchange, oxidation, epoxidation, dihydroxylation, carbonylation, and alkenation (Wittig or Horner-Wadsworth-Emmons reactions). (Molander, 2007) The boron itself is not protected from the Suzuki-Miyaura reaction, but can be used directly in the coupling transformation. Another class of tetracoordinate boron anions, [R—B(OH)3]−, has been reported in the context of purifying boronic acids for use in the Suzuki-Miyaura reaction. (Cammidge, 2006) As with the trifluoroboronate anion, the trihydroxyboronate anion is reactive in the Suzuki-Miyaura reaction.
These typical protection strategies for boronic acids each have some disadvantages. Boronic esters and boronic amides can protect the boron from a wide variety of reaction conditions, including Suzuki-Miyaura reaction conditions. However, the harsh conditions required for deprotection of boronic esters and boronic amides can cause undesirable side reactions with other functional groups. Trifluoroboronate anions also are unreactive in a wide variety of reaction conditions. However, this protection strategy does not allow for selective Suzuki-Miyaura reactions, since both protected and unprotected boron atoms will be eliminated in the coupling transformation.
It would be desirable to protect boronic acid groups in a wide variety of synthetic reactions, including the Suzuki-Miyaura reaction. Ideally, protected boronic acids would undergo deprotection under mild conditions with high yields. Such a system for controlling the reactivity of boronic acids could greatly expand the versatility of the Suzuki-Miyaura reaction or of other reactions of boronic acids.